Modern society depends upon electronic communication for many of its functions, where electronic communication may generally be divided between analog communications and digital, or discrete, communications. Digital communication has been the predominant form of communication in recent years.
Digital communication involves exchanging information using finite sets of signals. In practice, these signals may be electrical waveforms propagated from point to point, for example, along a controlled impedance transmission path of a printed circuit board (PCB). Other digital communication techniques use electromagnetic fields to propagate the information from one point to another through a free space medium.
Both free space and controlled impedance media, however, tend to modify information transmitted in an undesirable way before the information reaches the intended recipient. The transmission medium may be referred to as a channel that accepts a transmitted signal, S(t), and delivers an output signal, R(t), which is in general an imperfect rendition of signal S(t). The channel tends to corrupt the transmitted signal in one of two ways.
First, the channel may introduce noise into the transmitted signal from the electronic equipment used to perform the communication process. Other external noise processes, such as atmospheric electromagnetic noise and noise caused by other transmitted signals may significantly alter the transmitted signal. Second, the channel may distort the transmitted signal due to the physical limitations associated with the channel, e.g., the bandwidth limitations of a voice band telephone channel or the bandwidth limitations of a signal trace on a PCB.
Techniques exist today that attempt to minimize the effects of the channel on the transmitted signal. Some of these techniques lie within a process called source coding followed by a process of channel coding. Source coding is the process of accepting an original signal, such as discrete sequences, real numbers, or waveforms, and producing a sequence of symbols that represent the original signal. The sequence of symbols is generally characterized by a sequence of bits that represent the original signal in the best possible way given certain design constraints.
The channel coding process receives the source coded bit sequence and modulates the sequence according to a specific application. Example applications requiring specific modulation formats include Fibre Channel, Gigabit Ethernet, and Infiniband, to name only a few.
Regardless of the specific source and channel coding formats used, the resultant signal to be transmitted generally requires a greater bandwidth than was required by the original signal. Unfortunately, many channels exhibit a frequency dependent behavior, such that signal content at one frequency is attenuated differently than signal content at another frequency. In many cases, the channel tends to place a higher amount of attenuation on higher frequency content as compared to lower frequency content. Such a channel, therefore, is characterized by a low-pass frequency response, whereby content at the higher frequencies is distorted in magnitude to a larger degree than content at the lower frequencies.
The point at which the channel begins to significantly attenuate an input signal is called the cutoff frequency. The cutoff frequency is defined to be the frequency at which the magnitude of the output signal is attenuated by three decibel (dB) relative to a zero dB attenuation of the input signal. Frequency content that is higher in frequency than the cutoff frequency of the channel is attenuated in accordance with the transfer function, or frequency response, associated with the channel.
In order to compensate for the transfer function of the channel, an inverse to the channel transfer function is required at some point within the communication system such that the attenuation effects of the channel may be mitigated. One such placement within the communication system may be at the receiver of the transmitted signal. In such an instance, a receiver that counteracts the signal magnitude effects of the channel is said to equalize the effects of the channel. Thus, equalization seeks to render a substantially constant magnitude response across a specific bandwidth of interest regardless of a substantially non-constant frequency response provided by the channel.
Today's modulation formats, however, may require several gigabits to several tens of gigabits of bandwidth. The increasing bandwidths required by these modulation formats intensify the challenge to receiver hardware designers in providing receivers that not only are capable of receiving such wide bandwidth signals, but are also capable of equalizing the content of such wide bandwidth signals.
One such challenge is the implementation of a receiver design using standard glass epoxy PCBs. Each signal trace on the PCB is characterized as a transmission line, having a distributed reactance representative of a low pass filter having an upper frequency limit. As the bandwidth of the signals increases past this upper frequency limit, however, second order effects may cause a second corner frequency to exist, thus doubling the attenuation slope of the frequency response of the signal trace at the second corner frequency.
Each pole in the denominator of the transfer function of the signal trace causes a 20 dB per decade decrease in signal magnitude. That is to say, that if a particular signal trace provides a corner frequency at 100 megahertz (MHz) exhibiting a 3 dB decrease in signal magnitude, then a 23 dB decrease in signal magnitude is realized at 1000 MHz, or one frequency decade away from the corner frequency. In addition, second order effects may cause a second pole to exist at, for example, 5000 MHz, such that the magnitude of the signal begins to roll off at a 40 dB per decade slope at the second pole.
In order to offset the signal trace transfer function's pole effects, a receiving buffer is required to offset the attenuation caused by the signal trace. In particular, a receiving buffer is required that allows control of the receiving buffer's transfer function, such that zeroes in the numerator of the receiving buffer's transfer function may cancel the poles in the denominator of the signal trace's transfer function.
An apparatus and method that addresses the aforementioned problems, as well as other related problems, are therefore desirable.